Method for carburizing spinning nozzles composed of tantalum and alloys thereof



Apn] 29, 1958 K. RUTHARDT 2,832,710

METHOD FOR CARBURIZING SPINNING NOZZLES COMPOSED OF TANTALUM AND ALLOYSTHEREOF Filed March 1, 1956 2 Sheets-Sheet 1 Unite i METHOD F ORCARBURIZIN G SPINNING NOZZLES COMPOSED F TANTALUM AND ALLOYS THEREOFApplication March 1, 1956, Serial No. 568,831

5 Claims. (Cl. 148-131) The present invention relates to spinningnozzles for the production of synthetic filaments. It is primarilyconcerned with spinning nozzles for the manufacture of rayon and viscosewool, while, however, spinning nozzles for the manufacture of syntheticfilaments of other materials are not excluded. More specifically theinvention relates to spinning nozzles made of tantalum, and has for anobject to improve the qualities of the material constituting thenozzles.

This application is a continuation-in-part of application Serial No.240,774, filed August 7, 1951, now

abandoned.

Tantalum, owing to its high chemical resistance, is widely used for themanufacture of spinning nozzles. A disadvantage, however, consists inthe fact that tantalum is relatively soft, and that it is difiicult toharden-this metal without having to contend with other considerabledisadvantages. Thus for example, alloying only has led to brittlealloys, and those alloys which retain the characterizing desirablequalities of tantalum are unsuitable for being shaped by cutting tools.Out of various methods for surface hardening tantalum, only hardening byoxidation has hitherto found entrance into practice; this method,however, is unsuitable for many purposes and, to say the least, reducesthe utility value of the products because the material loses its glossymetallic surface. Thus for example, in one known method the completedtantalum spinning nozzles are heated for a short time in air, oxygen,nitrogen or carbon monoxide, at temperatures between 300 and 600 C. Inthis manner a surface hardening is obtained at the price of thedisadvantage that the tantalum becomes discoloured and its surface losesthe gloss. Hereby the quality of the spinning nozzles is impaired tosuch an extent as to cause them to get rapidly blocked during thespinning.

For this reason various suggestions have already been made aiming athardening tantalum in such a manner as to retain a glossy surface. Withthis object in view, the annealing has, for example, been effected inair or oxygen under reduced pressure. It is true that in this mannerhardening is obtained without the tantalum becoming dark; however, anincrease in brittleness occurs at the same time, the tantalum thushardened being apt to, crack and being quite unsuitable as material forspinning nozzles. Endeavours to carry out the hardening with nitrogenunder reduced pressure have likewise not yielded satisfactory results.Although these endeavors have succeeded in obtaining a bright, glossy,tough, hard and nonbrittle tantalum, the tantalum obtained has thedisadvantage that the nitride formed during the hardening reacts withthe lye of the spinning solution-presumably with formation ofhydrides-and owingto this in practical use the originally tough tantalumbecomes so brittle as to crack after some time.

It is therefore a further object of the present invention to producespinning nozzles of a tough and hard tantalum which is free from thedisadvantages referred States Patent to, that is to say which has aslight a colour as possible, and which is resistant to chemical attack.

With this and other objects in view, the present invention, in oneaspect, consists in partly converting the tantalum from which thespinning nozzle is made into tantalum carbide. In another aspect theinvention consists in a spinning nozzle consisting of tantalum whichcontains tantalum carbide, at least in its surface layer.

The transformation of the surface layer into tantalum carbide may be soconducted that the said layer consists wholly or preferably so that saidlayer consists partly of tantalum carbide.

While it is generally sufiicient for attaining the main object of theinvention to incorporate tantalum carbide in the surface layer, thegreatest improvement is obtained when the tantalum carbide extendsthrougout the cross-section of the spinning nozzle with the contenteither decreasing gradually towards the interior or extending throughoutthe cross-section in almost equal magnitude.

The tantalum carbide contents of the whole tantalum spinning nozzleshould be so high as to correspond to an overall carbon content of 0.01to 5%, and preferably to an overall carbon content of 0.05 to 0.5%.

Generally the-tantalum carbide content is paralleled by a content oftantalum oxide in quantities up to the indicated limits for the tantalumcarbide content.

A spinning nozzle according to the invention may have the form of any ofthe numerous spinning nozzles generally used in the art and described inliterature, for example that of the well-known cap or dish-like nozzleshaving a narrow edge at the upper end. The spinning apertures may bedistributed in the bottom of the nozzle in any of a great variety ofmanners; they may, for example, be arranged in rows extending radiallyof the bottom of the spinning nozzle. In a preferred form of the methodaccording to the invention, the conversion of the surface layers intotantalum carbide extends not only to the coherent surface of thespinning cap, but also to the internal surfaces of the fine spinningpassages.

Spinning nozzles which, in accordance with this invention, have acontent of tantalum carbide are distinguished by great hardness. Whilethe hardness of tantalum is normally, for example, 100 kg./mm. themicro-hardness of the surface layers of the novel spinning nozzles afterthe partial conversion of the tantalum into tantalum carbide is 200-600kg./mm. and preferably 300- 500 kg./mm.

When, in accordance with the preferred form of the invention the partialconversion of the tantalum into tantalum carbide is also efiected intothe interior of the cross-section of the spinning nozzle, an increase ofthe micro-hardness can there be likewise ascertained, the hardnesseither decreasing somewhat towards the interior or being nearly constantthroughout the cross-section according to the degree of conversion intotantalum carbide.

In order that the invention may be more readily understood, referencewill now be made to the accompanying drawings, in which:

Fig. l is an enlarged perspective view of a spinning nozzle according toone form of the invention;

Fig. 2 is an end view, the arrangement of the spinning passages beingindicated on a portion only of the bottom surface;

Fig. 3 is a diagram showing the variations of the microhardness acrossthe thickness of the bottom, and

Fig. 4 is a similar diagram showing the variations of the micro-hardnessalong a line extending inside the bottom, parallel to the bottom surfaceand extending radially of one of the spinning passages.

Referring now to the drawings, a tantalum spinning larged perspectiveview. In Fig. 2 the distribution of the spinning passages on part of thebottom of the spinning nozzle is indicated by way of example, althoughobviously the shape of the spinning nozzle and the distribution of thespinning passages are of no importance for the nature of the presentinvention. The production of the raw material and the shaping of thenozzle may be effected according to the normal practice.

This spinning nozzle may originally, for or o e, have a hardness of 100kg./mm. According to the invention the'hardness is increased topreferably between 400 and 500 kg/rnm. by partial conversion of thetantalum into tantalum carbide. The distribution of hardness in theinterior of the spinning nozzle may, for example, be as shown in Figs. 3and 4. In Fig. 3, the micro-hardness is plotted as a function of thedistance from the inner bottom surface of the spinning nozzle throughoutthe cross section of the spinning nozzle.

In the drawing the micro-hardness values at the inner surface of thenozzle bottom are indicated at 1 and those at the outer surface of thebottom are indicated at 2. it will be seen that the hardness graduallydecreases from a value of 480 kgjmm. at the two surfaces to about 400kg./mm. in the interior of the nozzle. These hardness values have beenobtained with the Zeiss micro-hardness tester.

It is important that the high hardness values found at the surfaceshould also be present in the interior surfaces of the spinningpassages. Fig. 4 shows the micro-hardness values as found in theneighborhood of a passage in an interior layer of the nozzle extendingparallel to the bottom of the nozzle, the micro-hardness values kg. persquare millimeter being plotted as a function of the distance measuredin It will be seen that the hardness values are generally of the orderof 350 leg/mm? and increase at the surface of a passage 3, 4 to valuesbetween 450 and 500 kg./mm.

The high values of the micro-hardness, more particularly on the surfaceof the spinning passages, are of importance since they are responsiblefor the unusual resistance which the spinning nozzles according to theinvention have been found to possess to grinding action, for examplethat caused in the case of matt rayon by pigments such as titaniumdioxide present in the spinning 1 substance.

Owing to their tantalum carbide content the spinning nozzles accordingto the invention also show considerably increased Vickers hardness,hardness values of 150 to 400 kg/mm. having been observed. This increaseof the Vickers hardness in conjunction with the abovementioned increaseof the micro-hardness results in a substantial improvement of themechanical strength of the nozzle body as compared with previously knowntantalum spining nozzles.

Although possessing these high hardness values, spinning nozzlesaccording to the invention have no tendency of becoming brittle. Theyalso are highly resistant to chemical attack, and more particularly arenot appreciably affected either byacid or alkaline mediums. The surfaceof the novel spinning nozzles shows a metallic gloss; when thetantalum-carbide content is low, the surface shows the light colour ofpure tantalum while in the case of higher tantalum carbide contents ithas a golden yellow metallic gloss.

Mainly due to their great hardness, the novel spinning nozzles have along life and give excellent spinning results; the spinning passages donot become clogged even after long use. 1

The spinning passages are produced before the partial conversion of thetantalum into tantalum carbide, thus obtaining the advantage that thespinning passages may be readily produced true to measure while themetal is soft, whereafter the hardness of the completed spinning nozzleis increased to a value which in practice would only with the greatestdifficulty permit a drilling of spinning passages.

Consequently, a spinnerette blank is first formed in the form of a disc,plate or cup, etc., from tantalum in a substantially pure state, or froman alloy of tantalum wherein the tantalum constitutes the major portionof the alloy. Thereafter spinnerette holes are drilled or otherwiseformed through the said metal blank and subjected to treatment withcarbonaceous gases as hereinafter more icularly described, whereby ahard glossy surface is imparted to the spinning passages or holes, saidpassage walls being very smooth and free of such deposits of carbonparticles or partially converted tantalum irregularities which wouldtend to interfere with the passage of a spinning mass therethrough.

A number of methods are available for the partial conversion of thetantalum spinning nozzles produced by customary methods into tantalumcarbide.

Treatment of tantalum in carbon monoxide between 300 and 600 C. leads inthe first place to the'formation of tantalum oxide. On the other handone readily succeeds in producing tantalum carbide if the treatment iseffected at higher temperatures, preferably between 700 and 1500 C., andat reduced pressures, preferably between 0.001 and mm. Hg, and moreparticularly between 0.001 and 1 mm. Hg with carbonaceous gases whichshould not contain any elementary oxygen. Suitable gases are, forexample, methane or carbon monoxide.

With treatment of a short duration in the abovedescribed manner only thesurfaces of the nozzles are partly converted into tantalum carbide andthus reach the indicated high hardness values.

If the treatment is extended for a longer period, the interior of thecross-section of the nozzle is, mainly due to diffusion, likewise partlyconverted into a tantalum carbide. In order to obtain this invaluablecondition, a longer duration of the annealing treatment at temperaturesbetween 700 and 1500" C. is recommended. By extending the annealingtreatment over a sufficiently long period, the tantalum carbide contentmay even be caused to be more or less uniformly distributed throughoutthe cross-section. In general, however, a distribution of hardnessapproximately corresponding to that shown in Figs. 3 and 4 will be foundsufficient.

If the tantalum subjected to the above-described partial conversion intotantalum carbide, in the known manner originally contains tantalum oxidein addition to metallic tantalum, the ultimate product will containtantalum oxide in addition to tantalum carbide without interfering withthe nature of the invention. The object of the invention is alsoretained when during the conversion of tantalum into tantalum carbide,tantalum oxide is also formed in a certain proportion, or when thetantalum obtains from its surface a certainoxide content by contact withthe air.

The following are examples for the above-described partial conversion ofthe tantalum into tantalum carbide:

I. A tantalum spinning nozzle is treated for sixty min utes in a carbonmonoxide atmosphere of 0.01 Torr. mm. Hg at 1000 C. The spinning nozzleobtains a very high surface hardness while retaining its metallic gloss;the Vickers hardness in this treatment already amounts to 250 kg./-mm.As mentioned above, it is of advantage to provide the spinning passagesalready prior to the treatment with carbon monoxide because the nozzleis then still in a soft condition, and also because if the invention iscarried out in this manner, the surface of the spinning passages willalso obtain a hard, chemically resistant carbide coating. V

II. Similar results are obtained if the spinning nozzle is annealed forapproximately thirty minutes at a temperature of 1200 C. in carbonmonoxide as free as possible from oxygen at a pressure of 0.005 mm. Hg.

The spinning nozzles obtained are resistant to chemical attack both bythe spinning solution and by the precipitating liquid.

What I claim is:

l. A method of making spinning nozzles comprising forming orificesthrough a spinning blank composed of metal taken from the groupconsisting of substantially pure tantalum and alloys thereof wherein thetantalum constitutes a major portion of the alloy, and thereaftertreating the spinning nozzle with carbonaceous gases at temperaturesbetween 700 C. and 1500 C. and pressures of 0.001 to 100 mm. Hg.

2. A method of making spinning nozzles comprising forming orificesthrough a spinning blank composed of metal taken from the groupconsisting of substantially pure tantalum and alloys thereof wherein thetantalum constitutes a major portion of the alloy, and thereaftertreating the spinning nozzle with carbonaceous gases at temperatures of700 C.1500 C. and pressures of 0.01 to 1 mm. Hg.

3. A method of making spinning nozzles comprising forming orificesthrough a spinning blank composed of metal taken from the groupconsisting of substantially pure tantalum and alloys thereof wherein thetantalum constitutes a major portion of the alloy, and thereaftertreating the spinning nozzle with methane at temperatures between 700 C.and 1500 C. and pressures of 0.01 to 1 mm. Hg.

4. A method of making spinning nozzles comprising forming orificesthrough a spinning blank composed of metal taken from the groupconsisting of substantially pure tantalum and alloys thereof wherein thetantalum constitutes a major portion of the alloy, and thereaftertreating the spinning nozzle with carbon monoxide at temperaturesbetween 700 C. and 1500 C. and pressures of 0.01 to 1 mm. Hg.

5. A method of making spinning nozzles comprising talum carbide contentis distributed throughout the cross section of the nozzle.

References Cited in the file of this patent UNITED STATES PATENTS VonBolton Aug. 25, 1908 Van Note Aug. 29, 1939

1. A METHOD OF MAKING SPINNING NOZZLES COMPRISING FORMING ORIFICESTHROUGH A SPINNING BLANK COMPOSED OF METAL TAKEN FROM THE GROUPCONSISTING OF SUBSTANTIALLY PURE TANTALUM AND ALLOYS THEREOF WHEREIN THETANTALUM CONSTITUTED A MAJOR PORTION OF THE ALLOY, AND THEREAFTERTREATING THE SPINNING NOZZLE WITH CARBONACEOUS GASES AT TEMPERATURESBETWEEN 700*C. AND 1500*C. AND PRESSURES OF 0.001 TO 100 MM. HG.